Seismographic energy sources and geophone detectors are used to obtain a profile of geophysical formations beneath the earth's surface and are particularly important in the exploration for new reserves of oil, gas, and other natural resources. The source provides energy to the earth in an impulsive or vibratory manner. Detectors are placed at some distance away from the source. By analyzing electrical signals provided by the detectors, the location of sought-after formations may be determined.
One common technique is to use a surface energy source and bore hole detectors. In this technique, a well bore is drilled in the earth. A detector or an array of several detectors is lowered into the well bore. A profile is obtained by actuating the surface source and recording the detector outputs. The source is then moved and another profile is taken. Multiple profiles are normally required to adequately understand a formation. This technique is known as vertical seismic profiling. One disadvantage is the need to move and reenergize the source for each profile - as a result, the source is usually mounted on a mobile platform. Another disadvantage is that the number of detectors in the well bore is limited and only one record from each detection is provided by actuation of the surface source. Also, surface rubble and structures tend to attenuate source energy, and thus the source must necessarily be larger than would otherwise be required.
Another technique known as reverse vertical seismic profiling (in which this invention may be desirable used) places a source in the well bore (called a downhole source) and an array of detectors on the surface. Multiple profiles may be more easily taken as the source is easily moved to another location by raising or lowering a cable that supports it in the well bore. An advantage of reverse vertical profiling is that a large amount of data is recorded from the array of detectors for each source energization.
One type of prior-art downhole source is known as an air gun. The air gun provides a rapid release of pressurized air to an outlet placed in fluid in the well bore. The rapid air release provides an impulsive seismographic energy wave. Several problems are associated with air guns. A tube wave or secondary harmonic energy is created when the primary energy wave reflects off the well bore bottom and then the fluid surface, and continues to oscillate between the surface and bottom. This tube wave distorts the energy transmitted to the earth. As multiple profiles are taken, the well bore fluid becomes aerated and thus more elastic, also distorting transmitted energy. Another problem is the need to feed pressurized air from the surface to depths commonly reaching ten to twenty thousand feet. Finally, the air gun is an impulsive source, providing energy in a wide bandwidth. It is often desirable to use bandlimited energy, as it provides a more easily interpreted profile.
U.S. Pat. No. 4,252,210 issued Feb. 4, 1981, to Sodich shows a downhole source. An elongate tubular expandable packer is placed in the well bore and rapidly expanded to create a seismic energy wave. However, the expansion of this device requires overcoming any ambient pressure in the well bore. This becomes a problem if the expandable packer is used in a deep well bore. Also, one must wait for the packer to collapse before moving it to another level in the bore.
U.S. Pat. No. 3,221,833 issued Dec. 7, 1965, to Malmberg and assigned to General Dynamics Corporation of New York, N.Y., is a seismographic source using pistons to engage a well bore. This device must also overcome the ambient pressure in the well bore to operate, is not collapsible in case of a valve malfunction, and if its valves malfunction, may become stuck in the well bore.